The use of crystals, such as quartz crystals, is well known for the purpose of controlling the frequency of electrical oscillator circuits. However, it is also well known that the natural resonant frequency of such crystals changes when the crystals are subjected to forces that cause acceleration. Thus, for crystal oscillators operating in environments subject to acceleration and vibration, such as in aircraft, it is desirable to have some method for reducing oscillator frequency shifts and phase noise caused by acceleration.
The known methods for reducing the acceleration sensitivity of crystal oscillators include both active and passive methods. Passive methods make no attempt to sense mechanical vibration or to dynamically change the output frequency of the crystal oscillator. Such methods typically use vibration isolators having pairs of crystals aligned in anti-parallel relationship to cancel the acceleration induced frequency shifts. In active methods, acceleration sensors and feedback networks are used to compensate for the acceleration induced frequency shifts by adjusting the oscillator frequency.
Although active and passive methods have been used for vibration compensation, currently available vibration compensation schemes have not been entirely satisfactory over a broad range of vibration frequencies. Thus, there is a need for a vibration compensation scheme for crystal oscillators that compensates for acceleration induced phase noise and frequency shifts in crystal oscillators over a wide range of vibration frequencies.